Commercially available DC-to-AC (DC/AC) converters, e.g., 12 Volt DC to 120 Volt AC, 60 Hz converters, frequently have difficulty starting various electrical devices, e.g., televisions, motors, etc., that draw heavy currents when the devices are first powered up. In general, when heavy start-up loads are applied, the converter DC link voltage may be loaded excessively and fall below a required value. This low link voltage, in turn, results in waveform distortion at the output of the DC/AC converter, which may result in suboptimal load starting performance. In commercially available DC/AC converters, when the link voltage of the converter falls during a high load situation, peak clipping of the output waveform of the converter usually results. Converters that are designed to maintain a root-mean square (RMS) voltage or average voltage typically respond to the falling peak voltage by increasing a gain factor to boost the non-peak portion of the waveform in an attempt to maintain the desired RMS voltage.
Unfortunately, the dV/dt (rise-time) of converters operated in this manner usually becomes abnormally high, which can cause problems for some types of loads, such as those that rectify alternating current (AC) and charge a capacitor. In addition, such converters may create significant harmonic content in the output waveform, which can be problematic for magnetic devices, such as transformers and motors. Typical DC/AC converters have used a DC-to-DC (DC/DC) converter to boost a system voltage, e.g., 14 Volts DC, to a voltage above the peak of a desired sinusoidal output voltage, e.g., 170 Volts for a 120 Volt AC system. In such DC/AC converters, while an associated inverter is generally capable of delivering more power than its continuous rating for a brief period of time, subject to thermal limitations, the DC/DC converter is often not capable of sustaining the desired link voltage with the increased power delivery. Consequently, such DC/AC converters have required that the DC/DC converter be oversized in order to start an associated load. Unfortunately, oversizing the DC/DC converter increases the overall cost of the DC/AC converter, which is undesirable.
What is needed is a technique for temporarily increasing the power capability of a DC/AC converter to better allow the converter to start certain loads.